Envenomation by the brown recluse spider, Loxosceles reclusa, is a known health problem. The consequences of such a bite vary from almost none to severe local necrosis. Rarely, severe systemic effects also occur. The long-term objective of this research is to determine the molecular mechanism by which the mammalian toxin from this spider's venom exerts its harmful effects. A secondary objective is to complete the structural definition of this toxin and its relationship to other mammalian toxins isolated from other spiders of the genus Loxosceles. The specific aims of this proposal include the final characterization of the charge forms of this toxin to insure that only those forms which are actually toxic will be used in later experiments. The essentiality for toxic effects of the sphingomyelinase activity which is associated with the mammalian toxin will be determined by a series of inactivation and chemical modification studies. If it is essential for both the local and systemic effects of the toxin, its substrate specificity will be further defined. The length of time the toxin remains in the victim's body and its route of clearance will be determined using labeled toxin. The time course of development of both local and systemic toxic symptoms will be determined. The involvement of complement in the systemic effects caused by the toxin will be examined. An in vitro system for examination of the potential interacting systems for the development of toxic symptoms will be evaluated. The mammalian toxins from the venom of the North America Loxosceles deserta and the South American L. laeta will be purified and compared to that of L. reclusa.